The present invention relates to a phase change type optical recording medium, and a method of overwriting the same, and a recording system used for such overwriting.
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for rewriting or overwriting. One typical rewritable optical recording medium is of the phase change type wherein the recording layer is irradiated with a laser beam to change its crystallographic state whereupon a change of reflectance by the crystallographic change is detected for reproducing. Optical recording media of the phase change type are of great interest since the optical system of the drive unit used for their operation is simpler than that for magneto-optical recording media.
Most optical recording media of the phase change type use Ge--Sb--Te base or chalcogenide materials which provide a substantial difference in reflectance between crystalline and amorphous states and have a relatively stable amorphous state.
When information is recorded on a phase change type optical recording medium, the recording layer is irradiated with a laser beam of power (recording power) high enough to bring the recording layer to a temperature higher than the melting point thereof. The recording layer is melted at spots with the recording power applied thereon, and then quickly cooled so that recorded marks of amorphous state can be formed. When the recorded marks are erased, on the other hand, the recording layer is irradiated with a laser beam having such a relatively low power (erase power) as to bring the temperature of the recording layer to a temperature higher than the crystallization temperature thereof but lower than the melting point thereof. The recorded marks with the erase power applied thereon go back to the crystalline state because they are slowly cooled down after heated to the temperature higher than their crystallization temperature. With the phase change type optical recording medium, it is thus possible to modulate the intensity of a single light beam for overwriting.
As a result of the inventors' studies, however, it has been found that in a phase change type optical recording medium comprising a recording layer based on Ge--Sb--Te compositions, initially formed recorded marks cannot fully be erased after the optical recording medium is stored for at least about 100 hours at high temperatures while the recorded marks remain formed on the recording layer. This is true of even the case where the recorded marks are overwritten. A failure in erasing the former recorded marks represents an error in the absence of post-recording check. When two overwriting cycles takes place continuously without allowing the recording medium to be stored at high temperatures, i.e., when the recording layer is irradiated twice with a laser beam having a power level greater than that of erase power, the recorded marks can fully be erased. It is thus possible to avoid such an error by check after overwriting, and rewriting. However, this offers a problem that much time is needed for writing.
Prevention of degradation of a phase change type optical recording medium due to storage in a high-temperature environment is typically proposed in JP-A 5-159369. The publication alleges that a dielectric layer, which is to be provided on a substrate together with a recording layer, is made up of a composite material comprising a chalcogenide compound, oxygen and carbon (C), whereby internal stresses occurring in the dielectric layer are so reduced that an adhesive force between the dielectric layer and the recording layer can be increased, with the result that reliability is maintained over a long period of time with neither delamination or nor cracking. Thus, the publication says nothing about an erasability drop found after storage at high temperatures, and about how it is avoidable.